Immune recognition of foreign antigens is primarily mediated at the cellular level by T lymphocytes. The receptor on T lymphocytes responsible for this recognition is comprised of a complex of at least eight polypeptide chains, two of which, the alpha and beta- chains, determine the specificity of the receptor. The ligand recognized is compromised of an antigenic peptide bound to a cell- surface protein encoded by the class I or class II major histocompatibility complex (MHC) genes. These complex receptor- ligand interactions are fundamental to the induction and maintenance of immunity, and an understanding of the mechanisms of these interactions is a essential basis for the study and treatment of immune-related diseases. As a model system for studying receptor-ligand interactions, a series of T cell clones specific for pigeon cytochrome c have been characterized in terms of the fine specificity for antigen and MHC determinants as well as the primary sequence of the variable portions of the expressed alpha and beta-chains. In addition, we will utilize rearranged V beta 17 gene segments in order to take advantage of a unique MHC specificity associated with virtually all T cells expressing this V-region as part of a TCR. The basis for studying T lymphocyte recognition mechanisms will involve the cloning and manipulation of genes encoding the alpha and beta- chains of the T cell receptor (TCR) from these clones. In a series of steps, both the structural basis for antigen/MHC specificity will be dissected by a systematic replacement of individual amino acids, and more extensive regions of the receptor polypeptide chains. These changes will be made by site-directed mutagenesis of cloned genes and transfected into T cells to examine the effects of different mutations on the specificity of the resulting receptors. The results will be interpreted using the three dimensional structure of the related immunoglobulin variable regions as a model. The goal of these studies is to understand which regions of the TCR are responsible for antigen and MHC specificities, and whether the regions responsible coincide with the complementarity determining regions that form the combining site in immunoglobulins. Based on these structural studies, gene encoding hybrid immunoglobulin/TCR proteins will be constructed in an attempt to produce a soluble molecule with the specificity of a TCR. The production of such a molecule will be informative with respect to the relationship between immunoglobulin structure and TCR structure, and for biochemical and structural analyses that are not possible with limited quantities of membrane molecules that are insoluble in aqueous solutions.